The present invention relates generally to fishing apparatus used within a well bore to free stuck pipe or tools. More specifically, the invention relates to an advance in well jars of the mechanical type as exemplified by the tools disclosed in the prior art.
In clearing an object stuck downhole in an oil well, it is common practice to employ a catching or retrieving tool, either an overshot or a spear, to obtain a hold on the stuck object, known as a fish, and a jarring tool, known as a fishing jar, which is capable of delivering one or more jarring or impact forces to the stuck object in an effort to free it and remove it from the well bore. Designs of conventional fishing jars are of two basic types, hydraulic or mechanical. A principal difference between the two types is in the method of locking and releasing a mandrel to cause the jar to impact or fire. Both types have certain advantages and disadvantages.
A fishing jar of the hydraulic type usually has a mandrel with an attached sliding valve that fits closely in a restricted bore in an outer housing. When a jarring force is required, tension is applied to move the mandrel relative to the housing. This movement provides a temporary delay before the mandrel is released to produce a jarring force. By increasing or decreasing initial tension applied to a jar, the resulting jarring force may be varied to some extent. When a mandrel is released, the energy stored in the stretched pipe string or other operating string to which the fishing jar is connected accelerates the jar mandrel rapidly to its fully extended position against a stop. The stop converts kinetic energy of the rapidly moving mass of the pipe string into an intense jarring force which is transmitted through an overshot or a spear to a stuck object or fish. The jarring force developed is greater than the original static tension applied through the fishing string.
Hydraulic fishing jars have an advantage over mechanical fishing jars in the ability to vary the jarring force while the tool is downhole. However, due to design limitations, extremely high hydraulic pressures developed during the delay period in some jars cause premature seal failure, leaving the jar inoperative. Also, viscosity change in hydraulic fluid due to temperature increase while jarring reduces the delay period in some hydraulic jars to the point that the jar becomes useless.
Mechanical fishing jars utilize a mandrel to compress a series of disc springs, also known as belleville washers, instead of using trapped fluid, to restrain the movement of the mandrel relative to the housing. A latch mechanism is set at a predetermined position to release the mandrel when the disc springs have been compressed by the mandrel to that predetermined point. As in a hydraulic fishing jar, when a mandrel is released, energy stored in the stretched fishing string accelerates the jar mandrel rapidly to its fully extended position against a stop. This sudden stop converts the kinetic energy of the rapidly moving mass of the fishing string into an intense jarring force, which is transmitted to the stuck object or fish. Conventional mechanical jar designs typically require a jar to be brought to the surface to adjust the latch release point and thereby to increase or decrease the impact force delivered by the mandrel to the fishing jar body.
U.S Pat. No. 4,919,219 of Taylor discloses a repetitive cam arrangement that allows a jar to be adjusted While it is downhole. Adjustments range from a minimum setting, through a progression of increased settings, to a maximum setting by repetitive upward and downward movement of the fishing string. When impact adjustment reaches a maximum setting, the cam arrangement is programmed to return to its minimum set position. Any setting increment may be located by the operator by repetitive movement of the fishing string. The remotely adjustable fishing jar disclosed by Taylor does not allow a fishing jar to be used initially at its maximum jarring force. Also, the jar disclosed by Taylor does not allow readily for calculation of a maximum jarring force because of friction. Sometimes it is difficult to tell which setting is engaged. The proper jarring force may fall between settings, one of which is too low in force and the other of which will not allow the jar to fire. The jar setting could become out of synchronicity with a ratchet type overshot causing the overshot to release the fish.
Taylor discloses a single lug latch assembly and, therefore, that jar is limited in surface contact area. Taylor discloses compression of disk springs upwardly. When the latch assembly releases the disk springs, the energy released by the springs imparts a downward force on the jar, thereby reducing the net upward force communicated to the fish. An accelerator may be required to increase the net upward force to a useful amount.
Accordingly, it is an object of the invention to allow a fishing string operator to determine a desired jar impact force for each impact of the jar, all while operating at draw works above the surface.
It is a further object of the invention to provide a remotely adjustable fishing jar that can be reset for each firing stroke and for any desired impact force within the capability of the fishing string to apply tension to the jar, all while the jar is downhole and controlled through an operating string by an operator at the surface.
It is a further object of the invention to provide a fishing jar that eliminates the need for an accelerator.
It is a further object of the invention to provide an adjustable fishing jar that allows an operator to apply a selected amount of tension to a fishing string to set in a mechanical memory of the jar a desired impact force that ranges continuously over the range of tension that can be applied and, if the jarring force is not sufficient to free the fish, to allow the selection of another, higher impact force to be set into the mechanical memory of the fishing jar.
Another object of the invention is to provide a method for retrieving an object stuck within a well bore by 1 remotely selecting successive jarring impact forces for each firing stroke while the jar remains downhole, all without the necessity of repetitively sequencing through a predetermined cam array.